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examples/lazyseqs.janet

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+# An example implementation of functional, lazy
+# sequences, as in clojure. The lazy seq is essentially
+# A lazy linked list, where the next value is a function
+# that must be called (realizing it), and the memoized.
+# Use with (import "./path/to/this/file" :prefix "seq.")
+
+(defmacro delay
+  "Lazily evaluate a series of expressions. Returns a function that
+  returns the result of the last expression. Will only evaluate the
+  body once, and then memoizes the result."
+  [& forms]
+  (def state (gensym))
+  (def loaded (gensym))
+  ~(do 
+     (var ,state nil)
+     (var ,loaded nil)
+     (fn []
+       (if ,loaded 
+         ,state 
+         (do
+           (:= ,loaded true)
+           (:= ,state (do ;forms)))))))
+
+# Use tuples instead of structs to save memory
+(def- HEAD 0)
+(def- TAIL 1)
+
+(defn empty-seq
+  "The empty sequence."
+  [] nil)
+
+(defmacro cons
+  "Create a new sequence by prepending a value to the original sequence."
+  [h t]
+  (def x (tuple h t))
+  (fn [] x))
+
+(defn empty?
+  "Check if a sequence is empty."
+  [s]
+  (not (s)))
+
+(defn head
+  "Get the next value of the sequence."
+  [s]
+  (get (s) HEAD))
+
+(defn tail
+  "Get the rest of a sequence"
+  [s]
+  (get (s) TAIL))
+
+(defn lazy-range
+  "Return a sequence of integers [start, end)."
+  @[start end]
+  (if end
+    (if (< start end)
+      (delay (tuple start (lazy-range (+ 1 start) end)))
+      empty-seq)
+    (lazy-range 0 start)))
+
+(defn lazy-map
+  "Return a sequence that is the result of applying f to each value in s."
+  [f s]
+  (delay
+    (def x (s))
+    (if x (tuple (f (get x HEAD)) (map f (get x TAIL))))))
+
+(defn realize
+  "Force evaluation of a lazy sequence."
+  [s]
+  (when (s) (realize (tail s))))
+
+(defn realize-map
+  "Evaluate f on each member of the sequence. Forces evaluation."
+  [f s]
+  (when (s) (f (head s)) (realize-map f (tail s))))
+
+(defn drop
+  "Ignores the first n values of the sequence and returns the rest."
+  [n s]
+  (delay
+    (def x (s))
+    (if (and x (pos? n)) ((drop (- n 1) (get x TAIL))))))
+
+(defn take
+  "Returns at most the first n values of s."
+  [n s]
+  (delay
+    (def x (s))
+    (if (and x (pos? n))
+      (tuple (get x HEAD) (take (- n 1) (get x TAIL))))))
+
+(defn randseq
+  "Return a sequence of random numbers."
+  []
+  (delay (tuple (math.random) (randseq))))
+
+(defn take-while
+  "Returns a sequence of values until the predicate is false."
+  [pred s]
+  (delay
+    (def x (s))
+    (when x
+      (def thehead (get HEAD x))
+      (if thehead (tuple thehead (take-while pred (get TAIL x)))))))